In such a trap, especially one installed in a basement or some other part of a dwelling, it is desirable to provide means for not only blocking the escape of sewer gases but also preventing a backup of water from the sewer to the trap. Under severe overflow conditions, as where a sewer is flooded in a downpour, it is desirable to rely for this purpose on a shutter more effective than a simple check valve to stop the flow. A trap with a dual seal, i.e. a check valve in the form of a ball float and a shutter closable by a handwheel, is known for example from German utility model No. 1,917,289. Such a construction, however, is relatively cumbersome and requires an enlargement of the drain in comparison with more usual traps. Even so, the assembly of shutter and float valve is not easily installed and is difficult to extract.